In the late 1920's, divided chamber combustion systems for two-valve type cylinder heads opened the way for small high-speed diesel engines with designs by Harry Ricardo, and Mercedes-Benz. Many forms of divided chamber combustion systems have been proposed, but the two forms identified above are still the main designs for current diesel engines. The Ricardo design has since been applied to overhead valve (OHV) cylinder heads and to overhead camshaft (OHC) cylinder heads. Recently, some newer engines have been introduced with OHC and three valves per cylinder (two intakes and one exhaust). The basic design of the Ricardo "Comet" Mk Vb pre-combustion system (also often referred to as "swirl chamber" system) used on these engines, however, remains essentially the same as the original 1939 version. A minor change has emerged in recent Japanese engines consisting in the reversal of the relative positions of the fuel injector and the glow plug to eliminate air flow interference within the precombustion chamber otherwise caused by the glow plug.
While the "Comet" system provided excellent performance when first introduced, under present, more demanding operating conditions, it has many thermodynamic problems. One problem is that combustion does not actually take place as originally understood. The original perception was that a secondary combustion process took place in the dual pocket combustion cavity ("swirl pockets") formed in the main combustion chamber under high swirl conditions. The present thinking is that this was not accurate. In order to understand why the original perception is untrue, one needs to analyze the whole combustion process and the thermodynamic conditions immediately proceeding combustion. Following is such an analysis.
During engine operation, as the piston moves upwards towards the top-dead-center (TDC) position at the end of the compression stroke, air in the reduced cylinder volume moves towards the precombustion chamber and the area in the combustion chamber occupied by the "swirl pockets" through a well known squish process in which air is literally squeezed between closely spaced portions of the piston surface and the fire-deck surface of the cylinder head. Since the above identified "swirl pockets" (or main combustion volumes) were located to one side of the piston, a relatively large squish area was created over the remaining flat piston top from which air discharge was required. The air is discharged from the squish area by pumping energy delivered by the piston. The pumping energy is dissipated in air turbulence and friction as the air moves through the ever decreasing volume defining the squish clearance space between the piston and the cylinder head's fire-deck surface. Resultantly, energy in the form of heat is transferred to the piston through its top surface as well as to the cylinder head through its fire-deck surface. All of this heat energy is wasteful and must be subsequently absorbed by the engine oil and coolant. Finally, the heat energy must be disposed of through the cooling system which uses up additional energy by being required to drive the water pump and the radiator fan.
As can be understood, wasted pumping energy decreases the energy of the compressed air in the combustion chamber thereby lowering its compression pressure and temperature. Although this is detrimental under all engine running conditions, it is most wasteful during cold cranking and cold engine operation because the top piston surface and lower cylinder head deck surface are cold and thus absorb a maximum quantity of energy from the air moving from the squish areas. A high temperature differential between the surfaces and the air generates energy losses and, as is well known, whatever energy is lost must be compensated for by increasing the nominal or design compression ratio of the engine so as to reach the proper compression temperature for auto-ignition of the fuel in the combustion chamber. This, apart from being very expensive from the manufacturing standpoint, forces the engine to operate when hot at a compression ratio higher than needed for ignition and produces a combustion resulting in an increase in the firing pressure and the friction of the moving components of the engine, which must be made correspondingly stronger, and heavier. The higher firing pressure also requires a stronger engine structure which increases the vehicle's weight and fuel consumption. Moreover, the increased friction increases the fuel consumption and emissions as well as engine wear.
In addition, pumping air during the compression process from the squish areas between the cylinder and top surface of the piston and into the swirl pockets formed in the piston creates turbulence or swirl in these pockets. Such swirl is desirable in gasoline and direct injection diesel engines because it occurs before TDC and serves to accelerate combustion. However, in an indirect (divided chamber type) engine, it is of no benefit because combustion initially takes place before TDC in the pre-combustion chamber which has its own separate swirl-generating mechanism. Thus, by the time burning gases are discharged from the pre-combustion chamber into the main combustion chamber, the piston is already past TDC when previously created squish activity is past. Also, this divided chamber type of engine accelerates the secondary combustion process in the main combustion chamber by the kinetic energy of the products of prechamber combustion which exit through the transfer passages and by the highly reactive unburned fuel and the temperature of the burning mass. Accordingly, it is a waste to expend energy in creating swirl or turbulence.
Research has indicated that pumping work is substantially reduced if the area of combustion in the main combustion chamber (previously referred to as "swirl pockets") is expanded and centrally positioned so that any air that must be moved from the now volumetrically reduced and better-distributed smaller squish areas travels a minimum distance to the centrally located pockets. Although some air movement must always occur, the proportion of air movement from the squish areas is minimized and resultantly the energy expended is minimized.
The actual combustion process in the "Comet" systems begins in the pre-combustion chamber when fuel is introduced. The interior of the chamber is highly turbulent and confines a hot mass of air within hot pre-combustion chamber walls. Inasmuch as all of the fuel is introduced in the pre-combustion chamber and such chamber holds only a small portion of the total combustion air mass, the air-fuel (A/F) ratio within the pre-combustion chamber can be very rich, particularly under high load operation. Therefore, only a portion of this fuel burns with the air at a roughly stoichiometric A/F ratio. The rest of the fuel, which is heated and well mixed with the air and already formed products of combustion dissociates into highly reactive radicals. As the burning mass is expelled from the pre-combustion chamber, secondary combustion begins in the main combustion chamber. It was previously thought that this secondary combustion process occurred between the products of prechamber combustion and the fresh air in the piston swirl pockets, at a higher level of swirl, beginning from about five degrees ATDC and continuing for forty-five to fifty degrees of crankshaft rotation after TDC. However, research confirms that this is not very accurate. In reality, as the discharge from the pre-combustion chamber enters the piston pockets after TDC, the piston is rapidly descending during an expansion stroke and any air originally in these swirl pockets migrates to the ever increasing clearance volume being created over the large piston squash area between the top of the piston and the cylinder head's fire-deck; area which had little air when the piston was at TDC due to the minimum clearance.
A single, centrally located and relatively small transfer passage from the prechamber discharges its products of combustion as a relatively high-velocity torch and by a considerable expenditure of energy (pumping work). There is little incentive for the discharge to enter the piston's swirl pockets because the kinetic energy and momentum of the burning mass is too great to effect a change in direction and there is no other force to cause it to change direction. Specifically, the splitter formed on the piston top between the downstream circular ends of the "swirl pockets", intended to redirect the torch to enter the swirl pockets and burn therein while swirling, has descended with downwards movement of the piston and thus is no longer effective for changing the flow direction from the prechamber's transfer passage. Accordingly, secondary combustion continues past the pockets along the transversal centerline of the cylinder and follows the mass of air which has migrated to the far side of the chamber. This creates very high temperatures and very high levels of heat transfer to the piston because of the energy level and agitation of the combustion products. As a result, along a transversal line running between the valve bridge, the center of the piston's upper surface, and a portion of the exhaust valve seat a very high thermal loading is applied which could result in piston failure at the base of the splitter, as well as valve failure. Controlling these conditions and inhibiting valve failure requires the use of expensive materials.
The above thermal load problems have been more pronounced since engines began to be turbo-charged as indicated in a technical article written by J. A. Stephenson entitled "High Speed Diesels", appearing on page 245 in the 1988 issue of "Automotive Technology International". Moreover, confirmation of the above analysis can be found in the book entitled "Internal Combustion Engine Fundamentals" published by McGraw Hill Publishing Company (1988), and authored by J. B. Heywood. FIGS. 10-4 on page 499 of this book were supplied by Ricardo and Co. to Professor Heywood of the Massachusetts Institute of Technology and show in color pictures a sequential series of combustion phases. By following the flame propagation in the pictures from prechamber through combustion in the main chamber, the pictures confirm that the true development of the combustion process occurs transversely downstream of the piston's center and not in the swirl pockets.
Another characteristic of engines using the "Comet" pre-combustion system is that they require very tight clearances between the piston top and the fire-deck and valves. This is necessary to avoid squandering of the chamber's clearance volumes and reducing the compression ratio. These engines already require very high compression ratios which are difficult to achieve and control during production. The need for high compression ratios is due to the fact that the surface area of the pre-combustion chamber volumes also lose a great amount of heat to the engine coolant. When that energy loss is added to the pumping losses by high velocity passage of gases through the transfer port during the compression stroke, plus the wasted squish energy and heat transfer from it, the total detracts from the potential pressure and temperature at the time of injection and increases a delay in fuel ignition. Of course, this problem is worse during cold-start cranking and operation when much of the heat of compression is lost to the cold surfaces forming the combustion chamber and the prechamber walls. To avoid a resultant compression temperature too low to start the engine, the nominal or design compression ratio (NCR) was typically increased to a level unnecessary for normal operation. This requires a judicious control of clearance volumes in the main combustion chamber where only so much space can be allocated to the squish area. The problem then becomes a "chicken and egg" situation because the high NCR and resulting low main combustion chamber clearance volumes force a tight mechanical clearance between the piston and the lower deck of the cylinder head and between the piston and the valve heads.
In some prior engines, the clearance may be less that 0.001 or 0.002 inch under hot running conditions. This clearance would be just enough so that the piston and the valves do not contact during the valve overlap period of the cycle. Also, the valve lift is decreased during the overlap period occurring near TDC at the beginning of the intake period. The minimized valve lift causes unnatural valve timing events not seen in other type of engines. Specifically, the Intake Valve Opening (IVO) is forced to occur at a later than thermodynamically acceptable place in the cycle. Also, the Exhaust Valve Closing (EVC) is designed earlier than thermodynamically acceptable in the cycle. The resultant short overlap duration and minimum valve lift produces poor air and exhaust flows which has dire thermodynamic consequences. Thus, recompression spikes can occur under high-load, high-speed conditions as the exhaust valve is almost or totally closed while the intake valve is not open enough causing exhaust gases to be trapped in the combustion chamber with no place to go as the piston approaches TDC. The resultant recompression is undesirable as it produces negative work or in other words extracts energy from the piston. This will limit the engine's power and high speed potential and increase fuel consumption, smoke, noise, and emissions. Worse yet, the recompressed gas typically expands back into the intake manifold when the intake valve is opened. Resultantly, the exhaust gas heats the intake valve, the intake port, and the intake manifold. This heating reduces the volumetric efficiency of the engine and the EGR-like effect under high-speed, high-load conditions is not a welcome addition to the cylinder charge. Since the exhaust re-ingested into the cylinder takes the place of clean air, it also further reduces the volumetric efficiency and is the main cause of increased smoke and reduced power output. Also, the smoke carries highly abrasive carbon particles to erode pistons and piston rings. Further the particles are carried into the lubricating oil, thus forcing frequent oil changes. The exhaust re-ingestion also increases the fuel consumption, emissions, and the cylinder's thermal loading. The hot recompressed exhaust gas also transfers heat energy to the bridge between the valves and to the piston and further increases their thermal loading.
The abnormal timing schedule begets later than thermodynamically correct Intake Valve Closing (IVC), especially as two-valve engines require high valve lift and duration. Earlier than thermodynamically correct Exhaust Valve Opening (EVO), which occurs for the same reasons, not only wastes expansion energy which otherwise would contribute to engine power but instead becomes wasted pumping energy, as exhaust products in the exhaust system which is overloaded by such higher-energy exhausts products, must be mechanically evacuated by piston movement. Power output and mechanical efficiency are reduced. Fuel consumption, emissions, smoke, exhaust temperature, and thermal loading on combustion chamber components are increased.
In all of these prior swirl type, divided chamber engines, the main response to the problem of cracked bridge portions between the valves consists of drilling holes, one per cylinder, transversely through the metal of the lower deck of the cylinder head. The holes start opposite the location of the pre-combustion chamber and run through the bridge and discharge in front of the cast boss for the pre-combustion chamber. The sole purpose of these holes is to cool the bridge and exhaust valves. The approach is expensive because, apart from the drilling process itself, it also requires a cast boss and extra metal in the lower deck of the cylinder head. Each hole must be of relatively large diameter to avoid the risk of a broken drill bit and high cylinder head scrappage rates. The transversal hole must be plugged on the outside to avoid coolant leakage and must register with another vertical passage reaching through the cylinder head gasket and the top deck of the engine block into the block's water jacket. The metal boss at the bridge's center must be thick enough to run the drill causing kinks or bulges to exist in the intake and exhaust ports that reduce the port's flow coefficients. The worst result is that the thickened (actually widened) drill boss over the bridge will typically force the valves to be smaller than otherwise possible. The smaller valves reduce the engine's air flow capacity and the power associated with it. Thermodynamically, the long passage also forces unnecessary cooling of the fire deck and, with its discharge right in front of the pre-combustion chamber, induces additional heat losses from it. These losses also increase the cooling system loads which then require a larger water pump and radiator for increased parasitic losses and extra manufacturing cost.
Applicant's U.S. Pat. No. 5,309,879 issued on May 10, 1994; U.S. Pat. No. 5,392,744 issued on Feb. 28, 1995; and U.S. Pat. No. 5,417,189 issued on May 23, 1995 disclose solutions to correct the above described valve event problems and allow design of the engines with a lower NCR and provide better start-ability and low speed operation. A solution to the basic thermodynamic problem is provided by eliminating the swirl pockets of the original Comet engines and creating a dual pre-combustion chamber associated with dual valve-relief pockets in the piston. These valve-relief pockets serve to provide valve clearance for valve opening at a partial lift position during the overlap period of the engine cycle. They also permit desirable valve timing events. The valve overlap period is desirably increased by an earlier IVO and later EVC so as: to eliminate the possibility of recompression spikes; to allow proper scavenging of the products of previous combustion; to improve the ability to fill the chamber with clean air; and to reduce thermal loading of the combustion chamber. Thus, by advancing the IVO and retarding the EVC, it is also possible to desirably advance the IVC and retard the EVO. The earlier IVC produces higher compression pressures during cranking and better start-ability. The thermodynamic improvement results in an increase in the effective compression ratio and increased trapped volumetric efficiency resulting from less blow-back of air from the cylinder into the intake manifold during the early stages of the compression cycle. The later EVO extracts more energy from the combustion gases and expels a mass of lower energy exhaust during the exhaust process. It also lowers the energy expenditure in carrying out the exhaust process; which reduces fuel consumption, emissions, and thermal loading.
The above identified patents also offer solutions for reducing high thermal loading on the valve bridge and on the top of the piston. For example, the '744 patent discloses a four valve cylinder head with a centrally located pre-combustion chamber incorporating four transfer passages. This design moves the secondary combustion from the center of the cylinder outward into the valve-relief pockets provided in the piston. The '879 patent discloses a four-valve cylinder head with a side located pre-combustion chamber with a piston top designed to spread out the secondary combustion to overlie a greater area of the piston's top surface but still retains a certain amount of the combustion at the center of the piston. The '189 patent, which is directly applicable to two-valve engines using the aforementioned "Comet" system, addresses the problem not only by performing most of the secondary combustion in the valve-relief pockets in the piston and allowing desirable valve timing events for generating improved thermodynamic results but, in addition, by using a funnel type transfer passage. This last design improves the air flow into the pre-combustion chamber which results in an increase in its air-filling characteristic as well as the amount of combustion taking place in the pre-combustion chamber. As a result, the amount of combustion taking place in the main combustion chamber is reduced and, thereby, the thermal load on the valve bridge and the piston is lessened. More importantly, however, the funnel design diffuses the torch of the products of the pre-combustion chamber from the transversal center of the cylinder and more into the valve pockets thus reducing the localized temperatures over the center-point of the piston. Although this design is an improvement, the single transfer passage still does not totally eliminate the major problem resulting from the high energy of combustion along the cylinder transversal centerline atop the piston and the high thermal loads associated with this direction. In other words, these previous designs do not eliminate the need for a transversal coolant passage as described above which is used to reduce the valve bridge temperature.
What is necessary to correct the aforedescribed consequences is a system that desirably directs the flow of burning gases from the pre-combustion chamber to desired dual locations in the combustion chamber. The subject application provides an improved divided chamber combustion system which inhibits concentration of the secondary combustion at the piston's center and redistributes the secondary combustion more evenly over the total crown surface of the piston. Thus, a more desirable thermodynamic valve timing can be utilized as described in my earlier patents. The improved design should provide a higher volumetric and mechanical efficiency and allow the modified engine parts to be bodily interchangeable with prior "Comet" or swirl-chamber type engines with two or three valves. The new design will also help eliminate the necessity for the costly drilled cooling passage through the valve bridge as well as allow straighter and smoother inlet and exhaust ports to the combustion chamber which in turn allows use of larger valves and exhibits a higher flow coefficient. Another result would be faster combustion through improved air utilization and potentially higher engine speeds both of which would increase power output and reduce the fuel consumption, gaseous emissions, smoke creation and engine noise.